Liquid crystal displays (LCDs) are widespread used in watches and clocks, photographic cameras, technical instruments, computers, flat TV, projection screens and large area of information devices. The information in many liquid crystal displays is presented in the form of a row of numerals or characters, which are generated by a number of segmented electrodes arranged in a pattern. The segments are connected by individual leads to driving electronics, which applies a voltage to the appropriate combination of segments to display the desired information by controlling the light transmitted through the segments. Graphic information or television displays may be achieved by a matrix of pixels, which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductors. More advanced addressing schemes use arrays of thin film transistors to control the drive voltage at the individual pixels. This scheme is applied predominantly to twisted nematic liquid crystal displays, but is also finding use in high performance versions of super twist liquid crystal displays.
Ideal display should show equal contrast and color rendering, looking on them under different angles, deviating from the normal observation direction. The different kinds of displays based on nematic liquid crystal, however, possess an angle dependence of the contrast. This is, at angles deviating from the normal observation direction, the contrast becomes lower and the visibility of the information is diminished. The materials commonly used in nematic LCD's are optically positively uniaxially birefringent, this is extraordinary refractive index ne is larger then the ordinary refractive index no; Δn=ne−no>0. The visibility of the displays under oblique angles can be improved by use of optical compensators with negative birefringence (Δn<0). In addition, the loss of contrast is caused by light leaking through the black state pixel elements at large viewing angles. In color liquid crystal displays, the leakage also causes severe color shifts for both saturated and gray scale colors. These limitations are particularly important for avionics applications, where copilot viewing of the pilot's displays is important. It would be a significant improvement in the art to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view.
Optical compensators comprising discotic nematic or monoaxial columnar phases which are oriented in the liquid crystalline state are disclosed in EP 0 656 559. The solid-discotic phase transitions are very high; therefore heating to temperatures above 200 DEG C. is necessary. This is an inconvenient procedure and may cause decomposition in the quite sensitive materials.
An improvement of using discotic materials for optical compensators are disclosed in U.S. Pat. No. 5,699,136. The materials comprise discotic nematic phases with negative optical birefringence consisting of molecular weight liquid crystal oriented in polymer matrix, including a large variety of chemical structures. All these phases must be oriented in the discotic liquid crystalline state, applying alignment layers or electrical or magnetic fields, after having transformed the materials in the liquid crystalline state at markedly elevated temperatures. The optical axis is inclined by an angle of 10 to 40° to the direction perpendicular to sheet surface.
A method for lowering the phase transition temperatures of the discotic nematic phases by at least 10 DEG C. is disclosed in EP 0 676 652 A2, however, the transition temperatures remain as high as 105 DEG C. or higher. This means, that this method also desires strong heating of the layers, connected with the said disadvantages.
Also a multilayer thin film compensator is used to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view. That multilayer compensator including a first plurality of layers, each having a first refractive index and a first thickness, alternating with a second plurality of layers, each having a second refractive index and a second thickness. The values of the first and second refractive indices and thicknesses are such that the phase retardation of the multilayer is equal in magnitude but opposite in sign to the phase retardation of the liquid crystal layer in its homeotropically aligned state over a predetermined range of viewing angles.
Organic dichroic dyes are a new class of materials currently gaining prominence in the manufacture of optically anisotropic films with desirable optical and working characteristics. Films based on these materials are formed by coating a liquid crystal (LC) aqueous solution of supramolecules formed by dye molecules on a substrate surface with the following water evaporation. The produced films are imbued with anisotropic properties either by preliminary mechanical ordering of the underlying substrate surface as described in U.S. Pat. No. 2,553,961 or by applying external mechanical, electromagnetic, or other orienting forces to the coating on a liquid crystal substrate material as described in U.S. Pat. Nos. 5,739,296 and 6,174,394.
New type of materials for manufacturing optical anisotropic films is known in the prior art. The same films are formed from lyotropic liquid crystal on based supramolecules. Substantial ordering of dye molecules in columns allows use of these mesophases to create oriented, strongly dichroic films. Dye molecules that form supramolecular liquid crystal mesophases are special. They contain functional groups located at a molecule periphery that determine the water solubility of the dye. Organic dye mesophases are characterized by specific structures, phase diagrams, optical properties and dissolving capabilities as described in greater detail in J. Lydon, Chromonics, in Handbook of Liquid Crystals, (Wiley VCH: Weinheim, 1998), V. 2B, p. 981-1007.
Anisotropic films characterized by high optical anisotropy may be formed from LLC systems based on dichroic dyes. Such films exhibit both the properties of E-type polarizers, due to light absorption by supramolecular complexes, and the properties of retarders. Retarders are films with phase-retarding properties in those spectral regions where absorption is lacking. Phase-retarding properties of the films are determined by their double refraction properties: different refraction indices in the direction of LC solution deposition and the direction orthogonal to the deposition direction. If high-strength dyes are used for the film formation, the films are also characterized by high thermal and photo stability.
Extensive investigations aimed at developing new methods of creating dye-based films through manipulation of deposition conditions are currently underway. Of additional interest is the development of new compositions of lyotropic liquid crystals. New LLC compositions may be developed through the introduction of modifying, stabilizing, surfactant and other additives to known dyes, thus improving film characteristics. More detailed discussions of these processes are provided in U.S. Pat. Nos. 5,739,296 and 6,174,394 and published patent application EP 961138.
It is goal of this invention to provide a simple reliable process for producing compensation plates (in particular: “negative A-plate”, “positive” and “negative C-plate”) for LCD of different types.